The present invention relates to an improvement in an automobile speed control system for controlling actual automobile speed to a desired automobile speed. More particularly, the invention relates to a memory circuit which provides a speed reference signal used to maintain constant speed control in an automobile speed control system.
Speed controls for automobiles are now in commercial use. In such controls, the actual speed of an automobile is compared with a preselected desired value to generate an error signal for controlling the engine throttle valve to a position at which the actual speed equals the desired preselected value. Generally speaking, automatic speed control of automobiles requires stabiliation of the speed control system. If the stabilization should not be sufficient, so-called hunting, i.e., overshoot or undershoot of actual speed of the automobile, occurs. Such hunting can be prevented by deteriorating the responsibility and accuracy of the speed control system.
However, hunting can be prevented without deteriorating this responsibility by employing negative feedback control of the throttle valve as shown in U.S. Pat. No. 3,381,771, issued May 7, 1968. In the invention of this patent, an actual speed indication signal from the tachometer generator of the automobile speedometer and a preselected desired speed indication signal from a speed set potentiometer are supplied to a comparator circuit to generate an error signal which controls the throttle valve position. The error signal is supplied to a differential amplifier circuit together with a throttle position feedback signal (negative feedback signal) from a feedback potentiometer coupled to the throttle valve. The output signal from the differential amplifier circuit is supplied to a vacuum modulator which in response thereto provides to a vacuum motor a vacuum pressure related to the output signal. The vacuum motor is mechanically ganged to the throttle valve and controls its position. Thus the output differential signal of the differential amplifier regulates the position of the throttle valve. The negative feedback of the throttle valve position prevents extra movement of the throttle valve toward opening or closing. Therefore, over-acceleration and -deceleration of the automobile are prevented. The desired speed set on the speed set potentiometer is altered by rotating a lever connected to the slider of the potentiometer.
Similar speed controls for automobiles are disclosed in U.S. Pat. Nos. Re. 27,324, issued Mar. 28, 1972; 3,477,346, issued Nov. 11, 1969; 3,485,316, issued Dec. 23, 1969; and 4,056,157, issued Nov. 1, 1977.
A negative feedback circuit, without a feedback potentiometer, and which compares a speed error signal with an actual automobile speed signal to generate a negative feedback signal, is disclosed in U.S. Pat. No. 3,952,829, issued Apr. 27, 1976. Also, a negative feedback circuit, without a feedback potentiometer, and which includes a circuit means for retarding the speed error signal to be supplied to the desired speed signal generator circuit as the negative feedback signal, is disclosed in U.S. Pat. No. 3,793,622, issued Feb. 19, 1974.
Pat. No. 3,485,316 shows a system to control automobile speed constant and includes a speed memorization capacitor to which a first electric signal, indicating actual speed, is applied, and a set switch to close the charging loop of the capacitor for memorization of the actual desired speed as seen in Pat. No. 3,485,316. Also, the memorized voltage level on the capacitor is supplied to a comparator as the desired speed indication signal. Generally speaking, there are interposed in the charging loop of the capacitor mechanical contacts, such as those of a reed relay which is energized by the set switch. The mechanical contacts of a reed relay have relatively high insulation resistance in the open state and prevent leakage of the capacitor. However, a reed relay is relatively bulky and expensive, and insulation resistances between contacts, as well as between housing and contacts, vary in every relay. Moreover, a reed relay develops a noise induction voltage on the relay coil when it is deenergized, and this noise may undesirably influence the operation of some circuit element. Recently, a field effect transistor (FET) has been employed for switching the charging loop of the capacitor, as seen in Patent No. 4,056,157.
The present invention relates to an improvement of a memory circuit employing a memory capacitor with an FET in the charging loop of the capacitor.